1. Field of the Invention
This invention relates to a distortion compensation apparatus. More particularly, it relates to a method and apparatus applicable to a high frequency power amplifier for transmission usable in particular in a portable telephone set.
2. Description of Related Art
In keeping up with increasing speed and increasing capacity in communication, the demand for linearity in a transmission power amplifier in a digital radio communication equipment is becoming more and more strict. This, however, obstructs improvement in the power efficiency in the power amplifier.
On the other hand, the continuous call time of a digital portable telephone set now in widespread use in general is becoming longer. Thus, in offering a new digital radio communication equipment to the market, its using time cannot be discounted in view of the competition power proper to a product. With this in view, the trend towards introducing the technique of distortion compensation for improving the efficiency is becoming brisk.
However, this technique is in need of a voluminous circuit scale and is extremely difficult to realize in a portable telephone set which features small size and lightness in weight. Moreover, because of the properties of the portable terminal, the environment in which the set is used id changed significantly, such that it is imperative to realize this distortion compensation by adaptive distortion compensation in order to follow up with these changes in environment. This poses a serious problem in conjunction with the demand for reduction in size. As a device for distortion compensation, there is known a technique of pre-distortion employing compensation means having opposite characteristics to those of the distortion of the power amplifier.
As this technique of pre-distortion, there are several reports as to adaptations of pre-distortion and adaptations of feedforward techniques. Several conventional techniques in the field of the adaptive secondary cell device used for the pre-distortion are hereinafter explained.
As a first conventional stricture, FIG. 1 shows a block diagram of a technique introduced e.g., in 1992. European Microwave Conference Vol. 22, pp. 1125 to pp. 1130, xe2x80x9cPower Amplifier Adaptive Linearization Using Predistortion with Polynominalxe2x80x9d.
Referring to FIG. 1, if non-linear input/output characteristics of a power amplifier (PA) 114, the distortion of which is to be compensated, are given by Vout=A(Vin), in-phase and quadrature signals I, Q of the input baseband to be input at an input terminal 111 may be calculated by a linearization comparator circuit 112 using a function H(I, Q) which linearizes A(Vin). The resulting I, Q signals are fed to a digital to analog converter (DAC) 113 and thereby analogized, while being converted into high frequency band signals which are input to a power amplifier 114. An output Vout of the power amplifier 114 is detected and output from an output terminal 115, at the same time as it is converted in a demodulator circuit 116 into baseband signals If and Qf. As for adaptive compensation for coping with changes in temperature, the linearization comparator circuit 112 compares the input signals I, Q to detection signals If, Qf to adjust a constant in a linearization function H so that a difference therebetween will be equal to zero. This operation is repeated until the difference is correctly equal to zero to set the constant in the function H (I, Q) ultimately to a proper value.
As another conventional structure, there is, for example, a technique disclosed in IEEE Transaction on Vehicular Technologies, Vol. 43, No. 2, May 1994, pp. 323 to pp. 332, xe2x80x9cAdaptive Linearization Using Predistortionxe2x80x9d. FIG. 2 herein shows a block diagram disclosed in this publication. In FIG. 2, those parts or components corresponding to those shown in FIG. 1 are denoted by the same reference numerals. A conversion table 124, such as a memory, is accessed for the input signals I and Q to effect data conversion to derive data Ixe2x80x2, Qxe2x80x2 capable of linearizing the power amplifier 114. These data Ixe2x80x2, Qxe2x80x2 are input to the power amplifier 114. An output Vout of the power amplifier 114 is detected and converted in a demodulator 116 to produce signals If, Qf. For adaptive compensation, there is provided an address generator 123 for comparing the input signals I, Q to detection signals If, Qf from the demodulator 116 to adjust the address for accessing the conversion table 124 so that a difference en will be zero. The address generator 123 is repeatedly adjusted until the difference en is correctly zero to optimize the address value which accesses the conversion table 124.
In the above-described conventional structures, the constant included in the linearization function or the address for accessing the linearization table is optimized. However, in these structures, repetitive operations are carried out using a feedback loop to reduce the difference. However, the feedback loop suffers a serious problem that the loop contains characteristics of the power amplifier 114 such that there is no guarantee that the convergence to an optimum value in stability will be realized at all times.
Moreover, the above-described conventional structures are in need of a demodulator in order to effect conversion of the power amplifier output into the baseband output. Since this demodulator is in general a quadrature demodulator, the circuit scale is voluminous.
It is therefore an object of the present invention to provide a method and apparatus for distortion compensation whereby the distortion component of a device such as a power amplifier can be compensated extremely readily.
It is another object of the present invention to provide a method and apparatus which is not in need of the demodulator to simplify the structure.
In one aspect, the present invention provides a distortion compensation apparatus for compensating distortion components generated in a device, including adaptive amplitude distortion compensation means for finding an amplitude difference between an envelope signal of an input signal supplied to the device and an envelope signal of an output signal of the device, outputting adaptive correction data of amplitude distortion against temperature fluctuations in the device based on cumulative results of integration of the amplitude difference and for adaptively compensating the amplitude distortion of the device using the adaptive correction data.
Preferably, the distortion compensation apparatus includes amplitude distortion correcting means for outputting correction data for correcting the amplitude distortion of the device based on the envelope signal of the input signal supplied to the device, and for controlling the gain changing processing of the device responsive to the correction data to correct the amplitude distortion of the device.
Preferably, the adaptive amplitude distortion compensation means sums the adaptive correction data to the correction data used in the amplitude distortion correcting means to effect adaptive compensation of the amplitude distortion of the device.
Preferably, the adaptive amplitude distortion compensation means includes first envelope detection means for detecting an envelope signal of an input signal supplied to the device, second envelope detection means for detecting the envelope component of an output signal of the device, subtraction means for finding an amplitude difference between the envelope component of the input signal as detected by the first envelope detection means and the envelope component of the output signal as detected by the second envelope detection means, cumulating means for integrating and cumulating the amplitude difference as obtained by the subtraction means and adaptive correction data outputting means for outputting adaptive correction data against temperature fluctuations in the device based on the cumulated results in the cumulating means.
Preferably, the distortion compensation apparatus includes adaptive phase distortion compensation means for outputting adaptive phase distortion correction data against temperature fluctuations of the device based on a phase different between an input signal supplied to the device and an output signal from the device and for adaptively compensating the phase distortion of the device using the adaptive correction data.
Preferably, the distortion compensation apparatus further includes phase distortion correction means for outputting correction data for correcting the phase distortion of the device based on an envelope signal of an input signal supplied to the device and for controlling the phase shifting processing of the device responsive to the correction data to correct the phase distortion of the device.
Preferably, the adaptive phase distortion compensation means sums the adaptive correction data to the correction data used in the phase distortion correcting means to adaptively compensate the phase distortion of the device.
Preferably, the adaptive phase distortion compensation means includes phase different detection means for detecting the phase different between an input signal supplied to the device and an output signal from the device and adaptive phase distortion correcting data outputting means for outputting adaptive phase distortion correction data against temperature fluctuations of the device based on the phase difference as detected by the phase different detection means.
In another aspect, the present invention provides a distortion compensation method for compensating distortion components generated in a device, including an adaptive amplitude distortion compensation step for finding an amplitude difference between an envelope signal of an input signal supplied to the device and an envelope signal of an output signal of the device, outputting adaptive correction data of amplitude distortion against temperature fluctuations in the device based on cumulative results of integration of the amplitude difference and for adaptively compensating the amplitude distortion of the device using the adaptive correction data.
Preferably, the distortion compensation method further includes an amplitude distortion correcting step for outputting correction data for correcting the amplitude distortion of the device based on the envelope signal of the input signal supplied to the device, and for controlling the gain changing processing of the device responsive to the correction data to correct the amplitude distortion of the device.
Preferably, the adaptive amplitude distortion compensation step sums the adaptive correction data to the correction data used in the amplitude distortion correcting step to effect adaptive compensation of the amplitude distortion of the device.
Preferably, the distortion compensation method further includes an adaptive phase distortion compensation step for outputting adaptive phase distortion correction data against temperature fluctuations of the device based on a phase different between an input signal supplied to the device and an output signal from the device and for adaptively compensating the phase distortion of the device using the adaptive correction data.
Preferably, the distortion compensation method further includes a phase distortion correction step for outputting correction data for correcting the phase distortion of the device based on an envelope signal of an input signal supplied to the device and for controlling the phase shifting processing of the device responsive to the correction data to correct the phase distortion of the device.
Preferably, the adaptive phase distortion compensation step sums the adaptive correction data to the correction data used in the phase distortion correcting step to adaptively compensates the phase distortion of the device.
According to the present invention, distortion components in the power amplifier can be compensated extremely readily using a method comprised of envelope detection without resorting to quadrature demodulation. Moreover, since the distortion component is detected by integration of an output input difference to effect distortion compensation, even the slightest distortion compensation can be detected.